Have you tried ebay? There seem to be hundreds of varied priced items in all sizes in ebay fancy dress shops these days. Mind you thats a lot of boobage to fit in a standard fitting!!
What kind of thing were you thinking of, any chance there is something you can put together yourself?
I'm a member of Craftster -
Craftster - Crafting Community & Forums: Crafts, Patterns, Projects & Craft Ideas
They're brilliant help there and loooads of people put together costumes, i'm sure you can get ideas and advice on there if thats the direction you fancy going. <3
Ps. I have a 'fat sz 8' friend too. My response when she moans is "SO WE CAN SHARE CLOTHES NOW!! 8D 8D 8D"
Soon shuts her up.